964 PROF. OWEN ON THE [NoV. 19, 



my conclusions on that question by additional or confirmatory evi- 

 dence from autopsy of the fabricators of the still existing polythala- 

 mous and siphoniferous Cephalopods. 



Having prevailed upon Mr. Cuming, subsequently to my Mono- 

 graph on the anatomy of Spirula in the ' Zoology of the Sama- 

 rang,' to permit me to test that anatomy by dissection of his unique 

 specimen, I proceed to point out such structural facts, confirmed by 

 that dissection, as demonstrate the ventral position of the marginal 

 siphon of the internal shell of Spirula — and to combine therewith 

 corresponding anatomical observations on Nautilus, which, with the 

 demonstrated affinity thereto of Ammonites, equally prove the 

 ventral position of the marginal siphon of the external shell of that 

 extinct genus and its allies. 



The dorsal aspect of a Cephalopod is determined by the position 

 of the brain and eyes, i. e. by that predominating part of the brain 

 which sends off the optic nerves. The ventral aspect is shown by the 

 respiratory funnel (Plate LX. figs. 3 and A,j, and gills, m). No mala- 

 cologist has questioned these conclusions. The proposition might be 

 simplified by stating that the funnel shows the "ventral side" of 

 the animal, and that the opposite one is the " dorsal side." 



Accordingly, all who have occupied themselves with the organiza- 

 tion of the Cephalopods have pointed out the singular reversed posi- 

 tions of the mandibles as compared with those in such vertebrate 

 animals as repeat the cephalopodic condition of a " beak," as, e. g. 

 Ghelonia and Aves. 



Instead of the dorsal or upper mandible being the largest and 

 longest, so as to overlap the ventral or under mandible, as in birds, 

 the dorsal mandible, h, figs. 3 and 4, is the smaller and shorter one, 

 and is underlapped by the larger and longer ventral mandible, i, in 

 all Cephalopods. 



So, likewise, the branchiae, ii. figs. 3 and 4, m, m, lie in the ven- 

 tral part of the pallial cavity ; and, in short, the several viscera 

 occupy similar relations to back and belly in both Tetrabranchiate 

 and Dibranchiate Cephalopods 1 . 



The aspects of the body being thus abundantly and unmistaka- 

 bly determined, they can be as unequivocally predicated of the shell 

 under whatever shape or proportion it may be present. 



The portion of the shell of Nautilus which extends from the 

 fundus of the last chamber, along the dorsal aspect of the mantle, 

 as from A to A', fig. 3, is the "dorsal wall" of such shell, and so 

 must be its continuation backwards. 



The portion of the shell of Spirula which bears the same relation 

 to the dorsal aspect of the body is the " dorsal wall" of the shell, 

 A, A', fig. 4 ; while that part of the last chamber which protects the 

 ventral side of the muscular and visceral mass which it contains, small 

 though it may be, is part of the "ventral wall" of the shell, B, and 

 so must be its continuation as far as it extends, viz. to the " proto- 

 conch " (fig. 5, a) or incipient nuclear chamber. 



1 See " Supplementary Observations on the Anatomy of Spirula" in Annals 

 and Magazine of Natural History for January 1879. 



